1. Field of the Invention
The present invention relates to a fan. More specifically, the present invention relates to a serial axial fan.
2. Description of the Related Art
Recently, the size and the thickness of electronic devices including personal computers are being progressively reduced. In conjunction with this progress, electronic components are mounted in a high-density on the inside of the electronic device, so that it is difficult for the heat generated in the electronic device to be released. A fan is typically used to release heat. Various kinds of fans such as axial fans and centrifugal fans are provided for the intended use. Among them, in an electronic device such as a server, a serial axial fan in which two or more axial fans are connected in series is often used.
In such a serial axial fan, in addition to the cooling performances including a large volume of air and a high static pressure, other performances such as low power and low noise are also required. In order to realize such performances, in each of the axial fans connected in series, the rotation of each impeller is required to be efficiently controlled. In controlling such fans, it is necessary to use a complicated circuit in which various electronic components are combined. Especially in the serial axial fan, it is necessary to independently control two or more axial fans. Accordingly, the rotation of each impeller is controlled by a circuit board disposed in a corresponding one of axial fans.
In some serial axial fans, respective axial fans are not required to have the same uniform performances, but are instead required to have different performances. In such a case, electronic components used in the respective circuit boards are sometimes different. In some cases, an electronic component which can be shared by the circuit boards is provided in each of the circuit boards individually, so that the cost is disadvantageously increased.
As for the serial axial fan, in order to enhance the noise characteristic and the characteristic of volume of air, an optimal rotational speed is set in each of axial fans. Each of the axial fans may, however, sometimes operate at a rotational speed which is different from the set rotational speed by the influence of the variety of components. In addition, because of the influence of the changes in operating environments, such as system impedances of the serial axial fan, the serial axial fan may operate at a rotational speed which is different from the set rotational speed.